FIG. 1 is a configuration view illustrating a conventional mobile communication system of Japanese Patent Application Laid-Open Hei 8-331153. In FIG. 1, reference numerals 1, 2, 3 and 4 designate a mobile station installed in a movable car and so on, low speed transmission means for transmitting communication data of a low transmission rate to a base station 5, high speed reception means for receiving communication data of a high transmission rate from the base station 5, and an up-link of a data communication channel, respectively. Reference numerals 5, 6, 7 and 8 designate the base station which radio communicates with the mobile station 1 belonging to a radio covered area, low speed reception means for receiving communication data of a low transmission rate from a mobile station 1, high speed transmission means for transmitting communication data of a high transmission rate, and a down-link of the data communication line.
An operation of this mobile communication system will be described below.
For example, when the mobile station 1 need transmit communication data to a communication apparatus (not shown) connected to a public communication network, the mobile station 1 receives a control channel of the base station 5 and transmits a call through the control channel allocated to the mobile station 1.
That is, the mobile station 1 transmits control information necessary for radio communication to the base station 5 through the control channel (for example, information to the effect that the mobile station 1 transmits communication data to the base station 5 at a low transmission rate, while the mobile station 1 receives communication data from the base station 5 at a high transmission rate).
When the control information is transmitted from the mobile station 1, the base station 5 transfers the control information transmitted from the mobile station 1 to a mobile switching center and so on (not shown). After the mobile switching center and so on allow a request of the mobile station 1, the base station 5 sends information to the effect that the request has been allowed to the mobile station 1 through the control channel.
Thus, the transmission rate of communication data transmitted from the mobile station 1 to the base station 5 is set to a low transmission rate, while the transmission rate of communication data transmitted from the base station 5 to the mobile station 1 is set to a high transmission rate. Therefore, asymmetrical radio communication can be achieved in which the up-link 4 differs from the down-link 8 in the transmission rate.
Since the conventional mobile communication system is formed as described above, the asymmetrical radio communication may be achieved in which the up-link 4 differs from the down-link 18 in the transmission rate. However, means for changing the transmission rate of communication data is not provided. Thus, once the transmission rate of communication data is set, even though a data amount is changed in accordance with passed time, the transmission rate cannot be changed to a transmission rate corresponding to the changed data amount. The conventional mobile communication system has the above-described problem.
The present invention has been made to solve such a problem. It is an object of the present invention to provide a mobile communication system in which after the transmission rate of communication data is set once, the transmission rate of communication data may be changed appropriately.